Bail if 3rd party cookies disabled. Closes #2999.

Chrome and FF18 currently will not allow any third-party to access localStorage
  or cookies in a third-party iframe. Safari is smarter and allows this for third-
  parties after the user interacts with their site (via throwing the popup).

  Added an exception for IE8, as it disallows cookie setting but will actually
  allow network requests to go through. Not thrilled about UA sniffing, though.

  Verified does the right thing on Chrome, FF18, IE8, and android 2.2 stock handset.

See also earlier discussion in PR 3401.